This invention is related to high-bit-rate digital subscriber line (HDSL) modules for telecommunications channel unit cards.
In the early days of telecommunications, a copper wire was used to carry a single information channel. Because the large majority of cost is in the materials and construction of the physical link, telephony engineers since have developed ways to pack multiple communications channels onto a single physical link. Frequency division multiplexing (FDM) and time division multiplexing (TDM) have been devised to multiplex multiple streams of analog and pulse code modulation (PCM) digital signals, respectively, into a single stream. For digital signals, the time division multiplexing hierarchy is represented as DS0 (Digital Signal) through DS4, where a DS0 is a single 0.064 Mb/s channel, a DS1 is 24 DS0s multiplexed together, a DS2 is 96 DS0s multiplexed together, a DS3 is 672 DS0s multiplexed together, and a DS4 is 4,032 DS0s multiplexed together. The hierarchy DS1 through DS4 also is referred to as T1, T2, T3, and T4 when using a copper medium for transmission.
A similar time division multiplexing scheme, based on International Telecommunication Union CCITT's G.700 Series Recommendations, uses a 32-channel format referred to as E1, or CEPT-1, where each channel corresponds to a DS0 (0.064 Mb/s) signal. Thus, a E1 signal is based on 32 DS0s multiplexed together, a E2 is 128 DS0s multiplexed together, a E3 is 512 DS0s multiplexed together, and a E4 is 2,048 DS0s multiplexed together.
A digital loop carrier (DLC) at a central terminal (CT) is a multiplexor which can multiplex multiple analog and digital signals from subscriber telephone lines into a single multiple digital signal such as a T1/E1 signal. A mirror DLC located at a remote terminal (RT) can decode the multiplexed T1/E1 signal into a form suitable for transmission over subscriber telephone lines. This data flow also occurs in the opposite direction from RT to CT.
Each DLC essentially is a special purpose computer containing standard control, memory, power, etc. components but also a number of interface cards (line cards) for converting analog and digital signals from subscriber telephone lines into formatted digital data signals. Different types of interface cards service different types of subscriber telephone lines. Examples of interface cards include POTS (Plain Old Telephone Service) cards for analog telephone lines, FOX cards for fiber optic lines, ISDN (Integrated Services Digital Network) cards for ISDN lines, and a number of other wideband service option cards. After the appropriate interface cards convert the subscriber telephone line signals into formatted digital data signals, a DLC at a central terminal constructs a single E1 signal by means of time division multiplexing. At a remote terminal, the DLC decodes the E1 signal to deliver formatted digital data back to the interface cards. Interface cards convert the formatted digital data into a form suitable for transmission over subscriber telephone lines. The most mature technology for transmitting over these lines (e.g., standard unshielded twisted pair) is HDSL. HDSL, which involves special electronics at both the central office and the customer premise, allows the provisioning of T1/E1 local loop circuits much more quickly and at much lower cost than through conventional means.